This invention relates to the field of optical transmission systems and, more specifically, to densely packed optical transmission systems.
The optical power transmitted through an optical transmission fiber of an optical transmission system is limited by fiber nonlinearity (e.g., self-phase modulation, SPM, cross-phase modulation, XPM, and four-wave mixing, FWM) in optical transmission systems. XPM and FWM become particularly important when optical channels are densely packed. More specifically, FWM, also referred to as four-photon mixing, limits the optical power transmitted through an optical transmission system with very small channel spacing. As the wavelength separation between channels in a WDM network is designed to be smaller, the severity of FWM penalties associated with closely spaced channels increases dramatically. This effect restricts the allowable wavelength separation of channels carrying data and, therefore, also limits the number of allowable channels within a WDM network.
Four-wave mixing occurs when two or more optical waves create a beat frequency whose oscillation modulates the refractive index of the optical fiber. This process generates sidebands that produce crosstalk in the neighboring channels and lead to signal impairments. The magnitude of the FWM impairment for a given fiber is a function of the wavelength separation between interacting channels, the power in each channel, and the phase matching efficiency.
Several methods have been suggested to reduce or minimize FWM impairments associated with closely located channels. For example, phase matching, which is related to the magnitude of the FWM efficiency, can be decreased through the use of a high dispersion fiber. Although high dispersion within a fiber would decrease the phase matching and correspondingly decrease the FWM impairments, high dispersion within a fiber imposes additional penalties such as pulse distortion over long distances. High-dispersion fibers may also additionally require the use of dispersion-compensating fibers to reduce accumulated dispersion.
Alternatively, because impairments from FWM are a function of the signal power in each channel, reducing the signal power or field amplitude for each channel has also been suggested. Because the power of the optical sidebands generated is proportional to the product of three individual component powers, FWM impairments can be greatly reduced by reducing field amplitudes. The signal power per channel, however, can only be reduced to a certain minimum level as dictated by the optical signal-to-noise ratio (OSNR) limit for the system. If the signal power per channel is decreased below the minimal level, the signal power compared to the noise power may not be sufficient to maintain an acceptably low bit error rate (BER). For traditional fiber systems with non-return-to-zero (NRZ) modulation and optical amplification, the signal power for each channel typically cannot be made sufficiently low to avoid FWM impairments for small channel spacing. Thus, traditional fiber systems avoid FWM penalties by increasing the wavelength separation between channels or use of a fiber type with large dispersion.
The above design modifications all suffer the same shortcoming, though. As one system parameter is adjusted to reduce FWM impairments, other system parameters are affected which prevent the simultaneous reduction of wavelength separation between channels and the reduction of FWM impairments. Thus, current design techniques limit the allowable minimum wavelength separation between channels.
The present invention advantageously provides a method and system for the suppression of nonlinear signal distortions, such as, self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM), associated with the propagation of a dense wavelength-division multiplexed (DWDM) optical signal or with the propagation of an optical signal via a low dispersion fiber.
In one embodiment of the present invention, a method for suppressing signal distortions in optical signals associated with nonlinearity in optical fibers includes propagating an optical signal through a transmission medium, the transmission medium comprising a sufficiently low dispersion such that a nonlinear distortion imparted on the propagating optical signal by the transmission medium is imprinted primarily on an optical field phase of the optical signal, and converting the propagated optical signal to an electrical signal such that the optical field phase information of the optical signal is not translated into the electrical domain. Alternatively, the method further includes electrically filtering each channel of said converted electrical signal at a respective intermediate frequency of said each channel.
In another embodiment of the present invention, a system for suppressing nonlinear distortions in an optical signal includes an optical oscillator for producing an optical oscillation signal, the optical oscillation signal being co-propagated with the optical signal, a transmission medium for propagating the optical oscillation signal and the optical signal, and an optical-to-electrical converter for converting the propagated optical oscillation signal and the optical signal to electrical signals, the optical-to-electrical converter comprising a bandwidth inclusive of all of the wavelengths of said optical signals.